1. Field of the Invention
The present invention relates to an AC motor driving circuit which uses an AC power supply as an input and drives an AC motor by using a DC power supply including a storage battery or a capacitor, and an electric car driving circuit using the driving circuit.
2. Description of the Related Art
For an AC motor driving circuit of this kind, there is previously known a circuit as shown in FIG. 12 (e.g. see JP-A-2004-112883).
FIG. 12 is a circuit diagram showing a first example of the prior art. In FIG. 12, a three-phase output of an AC motor M2 is converted into DC power by a voltage-type rectifier circuit 31 and the DC power is smoothed by a capacitor 13. The voltage-type rectifier circuit 31 has bridge-connected arms. Each of the arms has a switching element, and a diode back-to-back connected to the switching element. A DC chopper circuit 12, formed of a reactor L1 and switching devices Q1 and Q2 paired with their respective corresponding diodes D1 and D2, is provided between the smoothed DC power (between junctions N1 and N2) and a battery B so that electric power can be exchanged between the smoothed DC power and the battery B. Moreover, an AC motor M1 is driven by a voltage-type inverter 14 using the DC power. The voltage-type inverter 14 has bridge-connected arms each of which has a switching element (Q3 to Q8), and a diode (D3 to D8) back-to-back connected to the switching element. A control apparatus 30 controls the AC motor driving circuit based on torque command values (TR1 and TR2) and number of motor rotations (MRN1 and MRN2).
On the other hand, a circuit shown in FIG. 13 has been known as an example in which an output of an AC motor generator is directly converted into AC power by a matrix converter and the AC power is fed to an AC motor (e.g. see JP-A-2005-318731).
FIG. 13 is a circuit diagram showing a second example of the prior art. In FIG. 13, a matrix converter 38, which is composed of nine switches arranged as a three by three switch matrix (switches SAa to SCc), is used for achieving power conversion between motor generators MG1 and MG2. A voltage-type inverter 36 and a DC chopper circuit 32 are further provided for exchanging electric power with a battery 12. The voltage source inverter 36 is formed of a bridge circuit with each of switching devices Q31 to Q36 paired with their respective corresponding diodes D31 to D36 connected in inverse parallel therewith as each of six arms. The six arms are composed of two U-phase bridge arms 36U, two V-phase bridge arms 36V, and two W-phase bridge arms 36W. The DC chopper circuit 32 is formed of a reactor L and switching devices Q11 and Q12 paired with their respective corresponding diodes D11 and D12. A control apparatus 40 controls the AC motor driving circuit based on control signal from a controller 48.
In FIG. 12, it is however difficult to reduce circuit size because it is necessary to provide a large-capacity capacitor as a capacitor 13 for smoothing DC power. Moreover, because it is necessary to provide a reactor L1 in the DC chopper circuit 12, there is a problem that this respect leads to increase of circuit size.
On the other hand, in FIG. 13, it is also difficult to reduce circuit size because it is necessary to provide a DC capacitor C2 between the battery 12 and the motor generator MG2. Moreover, because it is necessary to provide a reactor L in the DC chopper circuit 32, there is a problem that this respect leads to increase of circuit size.
For example, a technique is disclosed in IEEJ Trans. IA, Vol. 126, No. 9, pp. 1161-1170, “Technical Trends of Direct AC/AC Converters” in which a current-type rectifier circuit and a voltage-type inverter are combined so that an AC power supply and an AC motor are connected to each other without connection of any capacitor to an output stage of the rectifier circuit. The combination of a current rectifier circuit and a voltage-type inverter is also referred to as indirect matrix converter.
This indirect matrix converter can perform AC/AC power conversion in the same manner as the matrix converter. For example, the matrix converter 38 shown in FIG. 13 can be replaced by the indirect matrix converter.
In AC/AC power conversion, use of a matrix converter or an indirect matrix converter permits the capacitor to be dispensed with. When a DC power supply is further provided in the circuit and the voltage of the DC power supply is lower than the inverter input voltage, it is however necessary to provide a chopper circuit designated by 12 in FIG. 12 or by 32 in FIG. 13 between the DC power supply and the voltage-type inverter in order to boost the DC power supply voltage. When such a chopper circuit is provided, it is necessary to provide a large capacitor in an output of the chopper circuit (as an input of the inverter). There is a problem that this leads to increase of circuit size.